01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.
02 - In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
03 - In a large bowl, beat butter and sugar until light and fluffy, about 3 minutes.
04 - Add eggs one at a time, beating well after each addition. Mix in vanilla extract.
05 - Alternately add flour mixture and milk to the butter mixture, beginning and ending with flour. Mix until just combined.
06 - Divide batter evenly between prepared pans. Smooth tops. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
07 - Cool cakes in pans for 10 minutes, then turn out onto wire racks to cool completely.
08 - Beat butter until creamy. Gradually add powdered sugar, milk, vanilla, and salt. Beat until fluffy.
09 - Divide icing into three bowls. Tint each bowl with a different color: purple, green, and yellow (gold). Mix until evenly colored.
10 - Place one cake layer on a serving plate. Spread a layer of icing (any color) on top. Add the second cake layer.
11 - Frost the top and sides with alternating stripes or sections of the three colored icings.
12 - Decorate with colored sugar and optional Mardi Gras decorations. Chill cake for 30 minutes before serving for cleaner slices.